A bomb blast tore through a bus in the Israeli city of Tel Aviv today. At least 15 casualties were reported, three of them seriously injured. Hamas claimed responsibility as tensions in the region increased dramatically.

Police said they were investigating whether the bomb had been planted on the bus or whether it was the work of a suicide attacker. An Israeli driver who saw the explosion said the vehicle was “completely charred inside.”

The attack will raise fears that a full-scale ground invasion of Gaza will begin, putting a possible truce in jeopardy. The Israeli bombardment of the Palestinian enclave continued today despite international efforts to finalise a ceasefire.

Foreign Secretary William Hague said: “The UK is deeply concerned at emerging reports of a bomb attack in Tel Aviv. Our thoughts are with the individuals and families affected. We condemn it unreservedly. This shocking violence further underlines the urgent need for a full ceasefire.”

As the crisis entered its seventh day, US secretary of state Hillary Clinton visited the West Bank and Cairo after talks with Israeli leaders in Jerusalem. A proposed truce failed to materialise last night following disagreements between the Israeli authorities and Hamas. As the bid to broker a deal was stepped up, Israel continued to launch air strikes and Hamas militants carried on firing rockets into Israel, with reports that a house in Be-er Tuvia was hit this morning. At least 136 Palestinians and five Israelis have been killed since the latest conflict began last week. Yesterday, the deadliest day so far, 20 Palestinians and two Israelis died.

Overnight, Israeli jets bombed more than 100 targets and destroyed the main ministry building of the Hamas-run government in the centre of Gaza City. The official Israeli Defence Forces spokesman tweeted: “This morning, the IDF targeted Hamas operatives in their hiding place in Gaza where they were building rockets. Direct hit was confirmed.” There were also reports that Israeli aircraft fired two missiles at the home of Isam al-Da’lis, adviser to Gaza’s Hamas prime minister Ismail Haniyah.

Before the talks, Israeli prime minister Benjamin Netanyahu said: “If there is a possibility of achieving a long-term solution to this problem by diplomatic means, we prefer that. But if not ... Israel will have to take whatever actions are necessary to defend its people.”

UN Secretary General Ban Ki-Moon today described the situation as “deeply alarming”.

In Beersheba, Ben-Gurion University was ordered by the army to close and evacuate its 20,000 students.